The invention is in the field of NMR and relates particularly to RF probe structures.
The central components of a modern high resolution liquid sample NMR probe include, at least one resonator for coupling RF (resonant) radiation to (at least one) resonating aggregation of nuclear spins of a sample. The sample is typically of elongate extension along an axis coincidental with the direction of the static polarizing magnetic field B0. The resonator imposes on the sample an RF magnetic field (B1) transverse to B0. The achievable homogeneity of B0 is spatially limited and the practicality of coupling RF power to the resonator through finite leads motivate the use of an RF shielding structure interposed between the leads of the resonator and the sample. The RF shield structure ideally limits RF coupling to the resonant spins located within a prescribed axial region of B0 homogeneity. In particular excitation of sample portions outside the desired region of carefully shimmed B0 homogeneity due to irradiation from the leads is a parasitic effect to be minimized by the shields.
The RF coil and shielding is subject to the deleterious effects of eddy currents arising from rapidly switched independent magnetic fields, such as magnetic gradient fields. Eddy currents induced in the coil and shields produce transient magnetic fields in opposition to the switched field inducing the eddy current. Inasmuch as these parasitic fields are particularly close to the sample, the B0 field homogeneity is degraded and the parasitic fields, with undesirable persistence, also degrade the relative timing of steps associated with a given NMR experiment.
In order to control/reduce eddy currents in RF coil and shield structures operating at room temperature, the prior art has incorporated slots into these conductors. See, for example, U.S. Pat. Nos. 6,008,650; 5,192,911; and WO 92/17799 all commonly assigned with the present invention. To similar effect, see U.S. Pat. No. 4,875,013. It should be appreciated that the eddy current problem is many times more deleterious with the RF resonator at cryogenic temperatures than that experienced with the RF resonator at room temperature. For the purpose of this work cryogenic temperature shall be understood to include temperatures substantially below ambient. Recent advances in NMR include very high Q probes operating at rather low temperature. Under such conditions, eddy currents effects are enhanced and their consequent deleterious effects require more rigorous suppression.
As employed herein, the RF resonator of saddle coil geometry is completely divided between longitudinally adjacent inductive members to provide two electrically separate loops disposed on opposite facing surfaces enclosing the sample space. Each loop, defining a window to the sample encompasses slightly less then 2xcfx80 (around the loop) to accommodate the two leads of each loop. The leads from both loops are disposed longitudinally in the same direction away from the central region (windows) of the loops. In the preferred embodiment the coil is excited by application of RF power to one loop with mutual induction coupling to symmetrically excite the opposite loop.
The shields comprise cylindrical conduction each slotted to provide azimuthal shield portions approaching xcfx80 in angular extent. These shield cylinders are coaxial with the RF coil with the inner axial extent preferably aligned with corresponding outer edges of the RF coil windows. The separate gaps between the loops of the RF coil are preferably aligned with the slots of the shields to provide transverse windows where double resonance experiments are contemplated.
The subject matter of the present application is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in conjunction with accompanying drawings wherein like reference characters refer to like elements.